A solar cell capable of converting sunlight into electric power has been noted as an energy source substituting for fossil fuel. Currently, examples of a solar cell, which began to be partially put to practical use, include a solar cell employing a crystalline silicon substrate, and a thin-film silicon solar cell. However, the former has a problem in high manufacturing costs of a silicon substrate, and the latter has a problem in that it is necessary to employ various kinds of semiconductor gases and complicated devices. Therefore, it is still high in manufacturing costs. Thus, an effort to reduce costs per output of electric power generation by achieving a high efficiency of photoelectric conversion has been made in both of the solar cells, but the above-mentioned problems have not been solved yet.
A dye-sensitized solar cell applying photoinduced electron transfer of metal complex has been shown as a new type of solar cell in Japanese Kohyo Patent Publication No. HEI 5(1993)-504023 and Japanese Patent No. 2664194. This dye-sensitized solar cell has a structure in that a photovoltaic layer is constituted between electrodes formed on each of two glass substrates by using a photoelectric conversion material and an electrolyte material. This photoelectric conversion material tends to have absorption spectrum in a visible light region by adsorbing a photosensitized dye. In this solar cell, the irradiation of light on the porous photovoltaic layer generates electrons, which are transferred to the electrode through an external electric circuit. The electrons transferred to the electrode are conveyed by ions in an electrolyte and returned to the porous photovoltaic layer via the opposite electrode. Electric energy is thus produced in such a manner.
A technique of a low-cost manufacturing method on the basis of this principle of operation is described in Japanese Unexamined Patent Publication No. 2000-91609, which technique is outlined. First, a glass substrate on which a transparent conductive film (electrode) is formed is prepared. A platinum conductive film (electrode) and a titanium dioxide colloid power-generating layer are formed on another windable flexible substrate to obtain a multilayer body. When or after this multilayer body is formed, the power-generating layer is impregnated with an electrolytic solution. It is conceived that this technique allows a single-unit organic solar cell.
Meanwhile, a dye-sensitized solar cell module, wherein plural dye-sensitized solar cells are connected in series, is shown in International Publication No. WO97/16838. Specifically, an individual dye-sensitized solar cell has a structure in that a titanium oxide layer, a porous insulating layer and a counter electrode are sequentially laminated on a glass substrate on which a transparent conductive film (electrode) subjected to patterning in the shape of a strip is formed. Also, a conductive layer of one dye-sensitized solar cell is disposed so as to contact with a counter electrode of an adjacent dye-sensitized solar cell, thereby allowing both of the solar cells to be connected in series.
However, the dye-sensitized solar cells described in Japanese Kohyo Patent Publication No. HEI 5(1993)-504023 and Japanese Patent No. 2664194 have the following basic structure: an electrolytic solution is injected between two glass substrates with a constant gap retained therebetween to make up the dye-sensitized solar cell. Accordingly, even though a solar cell with a small area can be experimentally manufactured, the application to a solar cell with a large area such as 1 meter square becomes difficult. With regard to such a solar cell, an enlargement in the area of one solar cell (unit cell) increases generated current in proportion to the area.
Then, a resistance component in a lateral direction of a transparent conductive film to be used for an electrode portion is extremely increased and, consequently, internal series electrical resistance as a solar cell is increased. As a result, there arises a problem in that a curve factor (fill factor; FF) in current-voltage characteristics during photoelectric conversion is decreased and photoelectric conversion efficiency is decreased.
It is conceived that the use of a flexible substrate allows high-speed production in Japanese Unexamined Patent Publication No. 2000-91609. One solar cell (unit cell), however, is simply made larger in area, whereby there arises a problem in that an increase in internal series resistance makes difficult to achieve a large area in the same way as the above.
In order to solve these problems, an integrated structure is conceived such as to contact a first conductive layer of a rectangular unit cell with a second conductive layer of an adjacent unit cell, which structure is employed for an amorphous silicon solar cell module composed so that an amorphous silicon layer is held between the first and second conductive layers. With regard to this structure, however, adjacent photovoltaic layers need to be formed with a certain gap so as not to contact with each other. Generally, the conversion efficiency of an integrated solar cell module signifies generation efficiency per module area. Thus, a large area of a gap brings no contribution of light onto the gap to electric power generation, so that module conversion efficiency becomes poor even though the conversion efficiency of a unit cell composing the module is high. The manufacturing costs per unit output also tend to become high. A method of manufacturing the module needs to be developed for contracting a gap between adjacent unit cells.
Generally, the amorphous silicon solar cell is subjected to scribing and integrated patterning by laser or the like, which these processes are applied to a dye-sensitized solar cell with difficulty. The reason therefore is that a photovoltaic layer of a dye-sensitized solar cell comprises a porous body in order to adsorb more dyes. A minute pattern can not be formed by laser or the like on such a porous body for the reason that a portion on which a minute pattern is formed is poor in strength. In addition, use of the laser raises manufacturing costs.
In order to solve these problems, a porous photovoltaic layer is formed by using a screen printing process in such a manner as the solar cell described in International Publication No. WO97/16838 shown in FIG. 5. It is, however, impossible to form a shape shown in FIG. 5 only by the screen printing process, and after the porous photovoltaic layer is formed, patterning is performed by laser, air jet or the like, so that the same problem arises for the same reason as the above. In FIG. 5, 51 denotes a transparent substrate, 52 denotes an intermediate layer, 53 and 57 denote gaps, 54 and 56 denote porous layers, 55 denotes an intermediate porous layer, 58 denotes a top cover for sealing electrical insulating liquid, and 59 and 60 denote terminals.
Further, the thickness of the porous photovoltaic layer in the dye-sensitized solar cell generally needs to be approximately 10 μm or more for adsorbing more dyes by a surface of an oxide semiconductor composing the porous photovoltaic layer. It is difficult that a second conductive layer is uniformly formed on the porous photovoltaic layer manufactured in the above-mentioned manufacturing method. For example, in the case where a second conductive layer of one unit cell is contacted with a first conductive layer of an adjacent unit cell, a decrease in the efficiency of the dye-sensitized solar cell results from an increase in resistance and, additionally, the occurrence of a short-circuit due to the extreme thinning of a second conductive layer on the side of a porous photovoltaic layer.
Meanwhile, a method is also conceived such that a resin or the like is printed with a predetermined pattern, which is utilized for forming a porous photovoltaic layer and a second conductive layer; however, this method offers complicated operation processes, so that manufacturing tact is deteriorated and costs are raised.